Combine first 7 ingredients in a large bowl.
Wrap 1 piece prosciutto around each shrimp. Thread shrimp and nectarines alternately on 8 (8-inch) skewers. Place skewers in a shallow dish; drizzle with 1/2 cup soy sauce mixture, turning to coat. Cover and refrigerate 30 minutes.
Preheat grill to medium-high heat (350° to 400°). Remove skewers from marinade; discard marinade. Grill skewers 3 minutes on each side or until shrimp are opaque. Serve with remaining soy sauce mixture. Garnish, if desired.